Fill out online application form on www.des.hec.gov.pk and a list of required documents will be automatically listed there. Anyhow following were attached in my case.
Original Secondary School Certificate
Original Higher Secondary School Certificate
Passport showing my stay for study in Germany
Enrollment certificate of University
Original transcripts of Degree
Original Degree
copy of Research Thesis
Supplement of degree
Original N.I.C of Pakistan

Remember HEC also needs confirmation about the genuiness of your degree from your university. HEC accepts such confirmation through Fax/E-mail, so make it sure that concerned department/examination branch is ready to sent this confirmation to HEC.
Feel free to ask any other question
Regards,